Augustine

United States

59.363°N, 153.43°W; summit elev. 1252 m

All times are local (unless otherwise noted)

Unrest continued at Augustine during 24 February to 3 March, with overall seismicity remaining low, but above background levels. Rockfalls and avalanches originating from the summit area continued to be recorded by the seismic network. Since inflation commenced on 10 February, a vertical change of 2-3 cm was measured by GPS (Global Positioning System). A thermal anomaly at the summit was visible on satellite and camera images, and incandescent avalanches were observed. All available information indicated that the lava dome continued to grow slowly. A plume composed of variable amounts of gas, steam, and small amounts of ash was emitted intermittently from the summit. Augustine remained at Concern Color Code Orange.

Geologic Background. Augustine volcano, rising above Kamishak Bay in the southern Cook Inlet about 290 km SW of Anchorage, is the most active volcano of the eastern Aleutian arc. It consists of a complex of overlapping summit lava domes surrounded by an apron of volcaniclastic debris that descends to the sea on all sides. Few lava flows are exposed; the flanks consist mainly of debris-avalanche and pyroclastic-flow deposits formed by repeated collapse and regrowth of the volcano's summit. The latest episode of edifice collapse occurred during Augustine's largest historical eruption in 1883; subsequent dome growth has restored the volcano to a height comparable to that prior to 1883. The oldest dated volcanic rocks on Augustine are more than 40,000 years old. At least 11 large debris avalanches have reached the sea during the past 1800-2000 years, and five major pumiceous tephras have been erupted during this interval. Historical eruptions have typically consisted of explosive activity with emplacement of pumiceous pyroclastic-flow deposits followed by lava dome extrusion with associated block-and-ash flows.